Vinyl chloride (VC) was administered via inhalation to groups of female hamsters, rats, and 2 strains of mice using various dosing regimens in an effort to compare the carcinogenic response. The different groups were exposed to VC for 6 hours/day, 5 days/week as follows: 0-6 months, 12-18 months, 12-24 months and 18-24 months following the start (animals were 8 weeks of age at start) of the experiment. VC related tumors were dependent on both the age of the animal during exposure and duration of exposure. In general, the younger the animal when exposed the higher the incidence of tumors when compared to groups exposed for the same length of time but starting with older animals. Exposures of longer than 12 months did not significantly increase tumor incidence.